From NHS choices :-
Chemotherapy
Chemotherapy is mainly used to treat prostate cancer that has spread to other parts of the body (metastatic prostate cancer) and which is not responding to hormone therapy.
Chemotherapy destroys cancer cells by interfering with the way they multiply. Chemotherapy does not cure prostate cancer, but can keep it under control and reduce symptoms (such as pain) so everyday life is less affected.
The main side effects of chemotherapy are caused by their effects on healthy cells, such as immune cells. They include infections, tiredness, hair loss, sore mouth, loss of appetite, nausea and vomiting. Many of these side effects can be prevented or controlled with other medicines, which your doctor can prescribe for you.